Lightning protection for electrically conductive or insulating skin and core for honeycomb structure

ABSTRACT

Lightning protection utilizing lightweight electrically conductive coatings with a conductance of approximately 220 to 350 mhos per square co-cured or secondarily bonded to the outer surface of thin skin composite structure. Selection of either 2 to 3 ounces per square yard or aluminum mesh of 1 to 1.5 ounces per square yard, depending on corrosion compatibility, satisfy the conductance requirement and has shown superior lightning protection over heavier weight conductive coatings.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of continuation-in-partapplication Ser. No. 08/926,767, filed Sep. 8, 1997, now U.S. Pat. No.6,086,975, which is a continuation-in-part application of pendingapplication Ser. No. 08/092,836 filed Jul. 16, 1993 which is acontinuation-in-part of Ser. No. 07/641,846, filed Jan. 16, 1991, nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the prevention of outer skin puncture of thinskin sandwich laminates by direct lightning attachment.

This invention relates to lightning protection systems and, moreparticularly, to such a system for protecting electrically conductivecomposite material aircraft structures from puncture that have thinouter skin structure or thin skin sandwich structure. The protectionmethod is to place a controlled electrically conductive layer outwardlyof the conductive skin layer and in electrical contact with theconductive skin so as to disperse the current from severe lightningdirect attachment, and that has means for preventing puncture of theouter skin surface and minimize damage to the structure.

2. Background Art

Conventional aluminum aircraft structures typically have inherentlightning protection. An aluminum skin structure has uniform andpredictable material properties, including the electrical properties.Thus, protection against skin puncture by lightning can be providedsimply by sizing the aluminum skin thickness to a range of 0.404 to0.060 inch. In areas of conventional aircraft structure where the skinis electrically non-conductive, such as radomes and aerodynamicfairings, metal bus bars can be applied to the exterior surface todirect the lightning currents to the aluminum structure.

Providing lightning protection for electrically conductive compositematerial structure, such as carbon fiber reinforced plastic, is muchmore difficult and complicated than for typical aluminum structure.Conductive composites are nonhomogeneous and are considerably lessconductive than aluminum. In thick skin structure, i.e., greater than0.10 inch, the skin may be thick enough to resist puncture but thestructure is still subject to severe skin surface damage, which can beminimized by applying relatively heavy metal or insulating coatings, ora combination in layers, to provide adequate protection as noted inprevious patents that deal with thick skin electrically conductivecomposite structure.

Intuitively, one would expect to use heavier metal meshes to protectthin skin electrically conductive composite structure. However, forelectrically conductive thin skin and sandwich composite structure, thestructure is easily punctured by lightning attachment even when theheavy metal mesh coatings are used. Using heavier weight coveringscomparable to aluminum skin thickness would provide adequate protectionbut would negate the weight saving advantages of composite structureover that of aluminum.

It is the object of this invention to provide a lightweight single layerprotection system that will provide surface protection and preventpuncture of thin skin electrically conductive composite aircraftstructure. Previously the primary considerations for the selection ofthe lightning protection material has been based more upon themechanical, corrosion and weight aspects of the material.

It is further the object of this invention to select the material basedprimarily on the electrical properties of the protection layer wherein anarrow window of an electrical property, that is surface conductivity,provides optimum protection for thin skin electrically conductivestructure. It is fortuitous that this leads to a lighter weight solutionfor thin skin conductive composite structure than the current methodsused to protect thick skin composite structure. By applying a lighterweight metal mesh that has a controlled electrical surface conductivitywhich is less than that used for thick skin composite structure, skinpuncture from severe lightning strike attachment can be prevented forthin skin electrically conductive composite structure.

The lighter weight conductive coatings would also provide protection forthin skin structure that is non-electrically conductive, with or withoutelectrically conductive core structure.

SUMMARY OF THE INVENTION

Critical features of the present invention include: 1) The selection ofthe mesh weight based upon the electrical surface conductivity toprovide a precisely controlled and efficient protection to preventpuncture of thin skin structure; 2) The heavier more conductive meshweights do not necessarily provide better lightning protection than thelightweight meshes; 3) For carbon fiber reinforced plastic skins, theelectrically conductive layer makes electrical contact with theelectrically conductive carbon fiber skin structure to make electricalbonding inherent in the structural makeup.

Another important feature is that the metal mesh layer also providesElectromagnetic Interface (EMI) shielding and when combined with theinner surface foil EMI shielding, a reduction in thickness of the innersurface foil can be made to minimize wrinkling.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a top plan view of an aircraft showing areas for incorporatinga co-cured or bonded outer conductive ply on honeycomb core or thin skincowl and fairings;

FIG. 2 is a side view of the aircraft of FIG. 1 also showing areas forincorporating a co-cured or bonded outer conductive ply on honeycombcore or thin skin cowl and fairings;

FIG. 3 is a more detailed showing of cowl and fairings showing areas forincorporating a co-cured or bonded outer conductive ply on honeycombcore or thin skin cowls and fairings, while the metal areas do notrequire this type of protection; and,

FIG. 4 is a key useful in showing the transport aircraft strike zones inthe transport aircraft of FIGS. 1 and 2.

DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred embodiment of this invention consists of the use of alightweight metal mesh with a controlled electrical surface conductivityof 220 to 350 mhos per square co-cured or secondarily bonded to theouter skin surface of thin structure to prevent puncture for thesimulated lighting strike test levels. For CFRP, a corrosion compatiblecopper mesh, mesh weight of 2 to 3 oz. per square yard with aconductance of 220 to 350 mhos per square is sufficient. Heavier weightcopper meshes, e.g. 5 to 6 oz. per square yard with a conductancegreater than 425 mhos per square, do not provide as much protection asthe lighter weight meshes with a conductance of 220 to 350 mhos persquare for thin skin structure. For corrosion compatibility withaluminum structure, aluminum meshes of 1 to 1.5 oz. per square yard areelectrically equivalent.

The transport aircraft strike zones of FIGS. 1 and 2 are interpretedthrough use of the key of FIG. 4. It should also be noted that zone 3also underlies all other zones.

FIG. 3 shows engine cowl and fairings with accompanying hatching codeinterpretation of areas that may incorporate a co-cured or bonded outerconductive ply on honeycomb core or thin skin cowl and fairings. Such aprotection system could be utilized for any type of lightweighthoneycomb structure (aluminum, fiberglass, CFRP) or thin skin structure.Although copper mesh has been hereinbefore described, other lightweightconductive coatings could be utilized as long as it provides the samelevel of surface conductivity and meets corrosion compatibilityspecifications.

What is claimed is:
 1. A thin skin structure, containing no honeycombcore structure, having, an outer skin surface comprised of carbon fibercomposite structure and a copper metal mesh having a mesh weight of 2 to3 oz. per square yard co-cured or secondarily bonded to said outer skinsurface.
 2. A thin skin structure, containing no honeycomb corestructure, having an outer skin surface comprised of fiberglasscomposite structure and a copper metal mesh having a conductance lessthan 425 mhos per square co-cured or secondarily bonded to said outerskin surface.
 3. A thin skin structure, containing no honeycomb corestructure, having an outer surface comprised of fiberglass compositestructure and an aluminum metal mesh of 1 to 1.5 oz. per square co-curedor secondarily bonded to said outer skin surface.
 4. A thin skincomposite structure having an outer surface including a conductivecoating having a conductance range of 220 to 350 mhos per square.